Various controlling arrangements for rolling stands are known. The most important controlling arrangements are roll gap controls and rolling force controls. A prerequisite for both controls is that the actuating element by means of which the roll gap of the rolling stand can be set is adjustable under load.
In the case of roll gap control, an actuating distance setpoint value is fed to a position controller. The actuating distance setpoint value is set such that the roll gap is suitably set. The actuating distance actual value is detected by means of a suitable detecting element and likewise fed to the position controller. From the values fed to it, the position controller determines a manipulated variable, on the basis of which the actuating distance of the actuating element can be changed, so that the actuating distance actual value is brought closer to the actuating distance setpoint value. The position controller outputs the manipulated variable to the actuating element.
During the rolling of the rolled stock, the rolling stand springs up on account of the rolling force exerted on the rolled stock. To compensate for this springing, it is known to detect the rolling force (more precisely: the rolling force actual value), to determine the springing of the rolling stand from the rolling force actual value and to correct the actuating distance setpoint value in such a way as to compensate for the springing of the rolling stand. If the rolling force increases, the actuating distance setpoint value is therefore changed in such a way that the correction of the actuating distance setpoint value counteracts the increase in the roll gap caused by the springing.
The controlling arrangement described above operates entirely satisfactorily if the rolls by means of which the rolled stock is rolled are exactly round and are mounted exactly centrally. However, these two conditions are not generally exactly ensured. There is therefore generally an eccentricity and/or an out-of-roundness. Only the eccentricity is discussed in more detail below. However, the problems entailed by out-of-roundness are equivalent to the problems entailed by eccentricity.
If, for example, the roll gap is reduced on account of an eccentricity, the rolled stock is rolled more strongly in the roll gap. An increased rolling force is required for this. If—in a way corresponding to the procedure described above for compensating for instances of springing of the rolling stand—the increased rolling force is interpreted as springing of the stand, the roll gap is reduced even further by the procedure described above, in addition to the reduction of the roll gap caused by the eccentricity. The eccentricity errors of the rolls are therefore imposed on the rolled stock to an increased extent. If the rolling force increases as a result of eccentricity, the actuating distance setpoint value must therefore be varied in such a way that the roll gap is opened up, in order to compensate for the eccentricity-induced reduction of the roll gap. The required variation of the actuating distance setpoint value in cases of eccentricity-induced rolling force changes is therefore diametrically opposed to the required changing of the actuating distance setpoint value that is attributable to other changes of the rolling force.
In the prior art, it is known in the case of a roll gap controller to determine the eccentricity of the rolls from the periodic fluctuations of, for example, the rolling force or the tension in the rolled stock upstream or downstream of the rolling stand under consideration, and to compensate for the eccentricity of the rolls by corresponding pre-control of the actuating distance setpoint value. Only the remaining fluctuation of the rolling force is regarded as springing of the rolling stand and is correspondingly corrected. It is of decisive significance in the case of this procedure that the changing of the actuating distance setpoint value brought about by eccentricity-induced changes of the rolling force on the one hand and brought about by changes of the rolling force due to other causes on the other hand are contrary. As already mentioned, the corresponding procedures are known. Purely by way of example, reference is made to U.S. Pat. Nos. 4,656,854 A, 4,222,254 A and 3,709,009 A.
In the case of rolling force control, a rolling force setpoint value and a rolling force actual value are fed to a rolling force controller. From the values fed to it, the force controller determines a manipulated variable, on the basis of which the actuating distance of the actuating element can be changed, so that the rolling force actual value is brought closer to the rolling force setpoint value.
In theory, an eccentricity of the rolls is not critical in the case of rolling force control. This is so because if, for example, an eccentricity briefly leads to a reduction in the roll gap, and consequently to an increase in the rolling force actual value, the actuating distance of the actuating element is changed in such a way that the roll gap is opened up, and therefore the rolling force actual value falls again.
In practice, however, the detection of the rolling force actual value is falsified by frictional forces which occur in the actuating element and in the rolling stand. Furthermore, the dynamics of the rolling force controls are too low, in particular at high rolling speeds, to compensate quickly enough for the eccentricity-induced rolling force fluctuations.
DE 198 34 758 A1 discloses a controlling arrangement for a rolling stand which has a force controller and a position controller. During the operation of the controlling arrangement, the force controller is fed a rolling force setpoint value and a rolling force actual value.
From the values fed to it, the force controller determines an actuating distance correction value. The actuating distance correction value and an actuating distance actual value of an actuating element are fed to the position controller. From the values fed to it, the position controller determines a manipulated variable, on the basis of which the actuating distance of the actuating element is changed. The manipulated variable is output to the actuating element.